Generally, an image forming apparatus includes a fuser that operates to fix an image on a document. The fuser typically includes of a pair of rollers, i.e., a heating roller and a press roller. The heating roller includes a heat source, and the press roller is pressed against the heating roller. As the document passes between the heating roller and the press roller, heat from the heat source fuses toner adhering to the document onto the document.
The wall of the heating roller is typically thin to reduce the time required for the heat source to warm up the heating roller. However, the thin wall of the heating roller has a small heat capacity so that the wall's ability to retain heat is diminished. Thus, at the start of a print operation when the rollers begin rotating, heat is transferred from the heating roller to the unheated press roller. Specifically, heat is transferred from the portions of the heating roller wall that come into contact with the cool press roller at a nip position of the rollers as the rollers rotate, resulting in an initial temperature fluctuation of the heating roller.
In a conventional image forming apparatus, a sensor that detects the temperature of the heating roller and activates the heat source when the temperature reaches a lower threshold is located at a fixed rotational angle beyond the nip position in a direction of rotation of the heating roller. Thus, when the rollers start rotating, the heating roller must rotate through the fixed rotational angle before a cool portion of the heating roller, that contacts the cool press roller at the nip position, reaches the sensor. The heat source is not activated until the sensor senses the cool portion of the heating roller. Therefore, activation of the heat source is delayed until the heating roller rotates through the fixed rotational angle so that the temperature of the heating roller is initially unstable. In other words, the heat source does not sufficiently compensate for the initial temperature fluctuation.
The insufficient compensation results in a risk that the heating roller will not be warm enough for proper printing, and the printing operation may keep the heating roller below the desired fixation temperature. Therefore, a need exists for better heating control of the start of a print operation.